Method and system for modifying a gate dielectric stack containing a high-k layer using plasma processing

ABSTRACT

A method and system for modifying a gate dielectric stack by exposure to a plasma. The method includes providing the gate dielectric stack having a high-k layer formed on a substrate, generating a plasma from a process gas containing an inert gas and one of an oxygen-containing gas or a nitrogen-containing gas, where the process gas pressure is selected to control the amount of neutral radicals relative to the amount of ionic radicals in the plasma, and modifying the gate dielectric stack by exposing the stack to the plasma.

FIELD OF THE INVENTION

The present invention relates to semiconductor processing, and moreparticularly, to a plasma processing method for modifying a gatedielectric stack containing a high-k layer.

BACKGROUND OF THE INVENTION

In the semiconductor industry, the minimum feature sizes ofmicroelectronic devices are well into the deep sub-micron regime to meetthe demand for faster, and lower power semiconductor devices. Thedownscaling of complimentary metal-oxide-semiconductor (CMOS) devicesimposes scaling constraints on the gate dielectric material. Thethickness of the conventional SiO₂ gate dielectric is approaching itsphysical limits. The most advanced devices are using nitrided SiO₂ gatedielectrics approaching equivalent oxide thickness (EOT) of about 1nanometer (nm) or less where the leakage current density can be as muchas 1 mA/cm². To improve device reliability and reduce electrical leakagefrom the gate dielectric to the transistor channel during operation ofthe device, semiconductor transistor technology is planning on usinghigh dielectric constant (high-k) gate dielectric materials that allowincreased physical thickness of the gate dielectric layer whilemaintaining a low equivalent oxide thickness (EOT). Equivalent oxidethickness is defined as the thickness of SiO₂ that would produce thesame capacitance voltage curve as that obtained from an alternatedielectric material.

Dielectric materials featuring a dielectric constant greater than thatof SiO₂ (k˜3.9) are commonly referred to as high-k materials. High-kmaterials may refer to dielectric materials that are deposited ontosubstrates (e.g., HfO₂, ZrO₂, HfSiO, ZrSiO, etc) rather than grown onthe surface of the substrate as is the case for SiO₂. High-k materialsmay incorporate a metal oxide layer or a metal silicate layer, e.g.,Ta₂O₅ (k˜26), TiO₂ (k˜80), ZrO₂ (k˜25), Al₂O₃ (k˜9), HfSiO (k˜5-20), andHfO₂ (k˜25).

Integration of high-k materials into gate stacks can require adielectric interfacial layer at the surface of the Si substrate topreserve interface state characteristics and form an interface with goodelectrical properties. However, the presence of an oxide interfaciallayer lowers the overall dielectric constant of the stack and,therefore, the oxide interfacial layer may need to be thin. The qualityof the interfacial oxide dielectric layer can affect device performance,as the oxide layer is intimately connected to the channel of thetransistor.

As-deposited high-k gate dielectric layers commonly contain pointdefects, vacancies or impurities that are incorporated into the high-klayers during the deposition process. These defects can be the source ofhigh leakage currents in the dielectric layer and may eventually beresponsible for premature failure of the dielectric layer and themicroelectronic device. Annealing procedures have been developed todecrease these point defects, however, high temperatures are usuallyrequired for maximum improvement, which can increase the thickness ofthe interfacial oxide layer.

SUMMARY OF THE INVENTION

A method and system are provided for modifying a gate dielectric stackby exposure to a plasma. The method includes providing a gate dielectricstack having a high-k layer on a substrate, generating a plasma from aprocess gas containing an inert gas and an oxygen-containing gas, or aninert gas and a nitrogen-containing gas, wherein the process gaspressure is selected to control the amount of neutral radicals relativeto the amount of ionic radicals in the plasma, and modifying the gatedielectric stack by exposing the stack to the plasma.

In one embodiment of the invention, the plasma can be generated from aprocess gas containing an inert gas and an oxygen-containing gas,wherein the process gas pressure is selected to increase the amount ofneutral oxygen radicals relative to the amount of ionic oxygen radicalsin the plasma. The plasma process modifies the gate dielectric stack byincreasing the dielectric constant of the high-k layer through reducingdefects in the layer, incorporating oxygen in the layer, or removingcarbon impurities or any other impurities from the layer.

In another embodiment of the invention, the plasma can be generated froma process gas containing an inert gas and a nitrogen-containing gas,wherein the process gas pressure is selected to increase the amount ofionic nitrogen radicals relative to the amount of neutral nitrogenradicals in the plasma. The plasma process modifies the gate dielectricstack by increasing the nitrogen content of the high-k layer.

The plasma processing system includes a plasma source for generating aplasma from a process gas containing an inert gas and anoxygen-containing gas, or an inert gas and a nitrogen-containing gas,wherein the process gas pressure is selected to control the amount ofneutral radicals relative to the amount of ionic radicals in the plasma,a substrate stage configured for supporting a substrate containing agate dielectric stack having a high-k layer on the substrate, whereinthe substrate stage is further configured for exposing the gatedielectric stack to the plasma, thereby modifying the gate dielectricstack, a substrate transfer system configured for transferring thesubstrate to and from the substrate stage, and a controller configuredto control the plasma processing system.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanied drawings:

FIG. 1A shows a gate dielectric stack containing a high-k layeraccording to an embodiment of the invention;

FIG. 1B shows a gate dielectric stack containing a high-k layer and aninterfacial layer according to an embodiment of the invention;

FIGS. 2A-2F are schematic diagrams of plasma processing systems formodifying a gate dielectric stack according to embodiments of theinvention;

FIGS. 3A and 3B show optical emission (OE) intensity as a function ofwavelength for an oxygen-containing plasma according to an embodiment ofthe invention;

FIGS. 4A and 4B show electrical characteristics for a plasma modifiedgate dielectric stack according to an embodiment of the invention;

FIGS. 5A and 5B show OE intensity as a function of wavelength for anitrogen-containing plasma according to an embodiment of the invention;

FIG. 6A shows nitrogen concentration profile in a gate dielectric stackas a function of plasma conditions and as a function of layer depthaccording to an embodiment of the invention;

FIG. 6B shows nitrogen concentration profile in a gate dielectric stackas a function of plasma exposure time and as function of layer depthaccording to an embodiment of the invention; and

FIG. 7 is a flow chart for modifying a gate dielectric stack accordingto an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1A shows a gate dielectric stack containing a high-k layeraccording to an embodiment of the invention. The gate dielectric stack 1contains a substrate 10 and a high-k layer 30 on the substrate 10. Thesubstrate 10 can, for example, be a semiconductor substrate, such as aSi substrate, a Ge-containing Si substrate, a Ge substrate, or acompound semiconductor substrate, and can include numerous activedevices and/or isolation regions (not shown). The substrate 10 can be ofn- or p-type, depending on the type of device being formed. The high-klayer 30 can, for example, be a metal-oxide layer or a metal silicatelayer, for example Ta₂O₅, TiO₂, ZrO₂, Al₂O₃, Y₂O₃, HfSiO_(x), HfO₂,ZrSiO_(x), TaSiO_(x), SrO_(x), SrSiO_(x), LaO_(x), or LaSiO_(x), or acombination of two or more thereof. The high-k layer 30 can, forexample, be about 3 nm thick.

FIG. 1B shows a gate dielectric stack containing a high-k layer and aninterfacial layer according to an embodiment of the invention. The gatedielectric stack 1 contains a substrate 10 and a dielectric layer 40that includes an interfacial layer 20 on the substrate 10 and a high-klayer 30 on the interfacial layer 20. The interfacial layer 20 can, forexample, contain an oxide layer (e.g., SiO_(x)), a nitride layer (e.g.,SiN_(x)), or an oxynitride layer (e.g., SiO_(x)N_(y)).

In one embodiment of the invention, the inventors have identified aplasma process for modifying gate dielectric stack 1 in FIGS. 1A and 1Bby exposing the gate dielectric stack 1 to an oxygen-containing plasmaat a high process gas pressure (high-pressure plasma). The high-pressureplasma contains increased amount of neutral oxygen radicals (excitedoxygen species) relative to the amount of ionic oxygen radicals,compared to low-pressure plasma. Modifying the gate dielectric stack 1using a high-pressure oxygen-containing plasma can include increasingthe dielectric constant of the high-k layer 30, reducing the amount ofcarbon impurities in the layer, reducing defects in the layer that giverise to high leakage currents or other electrical degradation aspects,or increasing the oxygen-content of the layer 30. Furthermore, thehigh-pressure plasma process minimizes growth (thickness) of theinterfacial layer 20 compared to high-temperature thermal oxidationprocesses and low-pressure plasma processes, which also have a higherconcentration of ionic oxygen radicals relative to neutral oxygenradicals.

Oxygen-based plasmas can primarily contain two types of oxygen radicals:ionic oxygen radicals (e.g., O₂ ⁺) and neutral (metastable) oxygenradicals (e.g., O*). According to an embodiment of the currentinvention, the amount of neutral oxygen radicals in a plasma, relativeto the amount of ionic oxygen radicals in the plasma, can be increasedusing high process gas pressure, for example, pressure between about 0.5Torr and about 5 Torr. In another embodiment of the invention, the gaspressure can be between about 1 Torr and about 3 Torr, and can be 2Torr. The process gas can contain an oxygen-containing gas including O₂,O₃, H₂O, or H₂O₂, or a combination of two or more thereof, and an inertgas including He, Ne, Ar, Kr, or Xe, or a combination of two or morethereof. In one embodiment of the invention, the process gas can containAr and O₂. In one embodiment of the invention, the ratio of the inertgas to the oxygen-containing gas can be between about 20 and about 5.For comparison, low-pressure plasma processing that utilizes a processgas pressure between about 10 mTorr and about 200 mTorr contain higheramount of ionic oxygen radicals relative to neutral oxygen radicals.

In another embodiment of the invention, the inventors have identified aplasma process for modifying the gate dielectric stack 1 in FIGS. 1A and1B by exposing the gate dielectric stack 1 to nitrogen-containing plasmaat a low process gas pressure (P˜200 mTorr). The plasma containsincreased amount of ionic nitrogen radicals (e.g., N₂ ⁺) relative to theamount of neutral nitrogen radicals (e.g., N₂*), compared to a highprocess gas pressure plasma (P˜800 mTorr).

Modifying the gate dielectric stack 1 using a low-pressurenitrogen-containing plasma increases the nitrogen-content of the high-klayer 30 while minimizing growth of the interfacial layer 20, therebyallowing better dielectric thickness scaling. Furthermore, the nitrogencontent of the high-k layer 30 increases with increasing plasma exposuretime. Low-pressure nitrogen-containing plasma minimizes growth of theinterfacial layer 20 compared to high-temperature thermal nitridation(nitrogen-incorporation) processes that use N₂O or NO gases and resultin interfacial nitridation but limited nitridation of the high-k layer30. Thermal nitridation processes using NH₃ also result in interfacialnitridation but limited nitridation of the high-k layer 30, and may needadditional annealing steps to reduce hydrogen (H) content of the high-klayer 20. Also, plasma nitridation processes using high-pressurenitrogen plasma result in increased interfacial nitridation, and lessnitridation of the high-k layer 30.

According to an embodiment of the current invention, the amount of ionicnitrogen radicals in a plasma, relative to the amount of neutralnitrogen radicals in the plasma, can be increased using low-pressureplasma. The process gas pressure can, for example, be between about 10mTorr and about 400 mTorr. Alternately, the gas pressure can be betweenabout 50 mTorr and about 300 mTorr, and can be 200 mTorr. The processgas can contain a nitrogen-containing gas including N₂ or NH₃, or acombination thereof, and an inert gas including He, Ne, Ar, Kr, or Xe,or a combination of two or more thereof. In one embodiment of theinvention, the process gas can contain Ar and N₂. In one embodiment ofthe invention, the ratio of the inert gas to the nitrogen-containing gascan be between about 20 and about 500.

In another embodiment of the invention, the gate dielectric stack 1 inFIGS. 1A and 1B can by modified by exposure to a high-pressureoxygen-containing plasma (i.e., pressure between about 0.5 Torr andabout 5 Torr) and, subsequently, the resulting modified gate dielectricstack can be exposed to a low-pressure nitrogen-containing plasma (i.e.,pressure between about 10 mTorr and about 400 mTorr).

In yet another embodiment of the invention, the gate dielectric stack 1in FIGS. 1A and 1B can by modified by exposure to a low-pressurenitrogen-containing plasma and, subsequently, the resulting modifiedgate dielectric stack can be exposed to a high-pressureoxygen-containing plasma.

FIGS. 2A-2F are schematic diagrams of plasma processing systems formodifying a gate dielectric stack according to embodiments of theinvention. It is to be understood that the plasma processing systemsdepicted in FIGS. 2A-2F are shown for exemplary purposes only, as manyvariations of the specific hardware can be used to implement processingsystems in which the present invention may be practiced, and thesevariations will be readily apparent to one having ordinary skill in theart. Like reference numerals are used to refer to like parts.

In FIG. 2A, the plasma processing system 100 includes a process chamber110 having a pedestal 112 for mounting a substrate stage 120 thatsupports a substrate 125 and exposes the substrate 125 to the plasmaprocessing region 160. The substrate stage 120 can be further configuredfor heating or cooling the substrate 125. The plasma processing system100 further includes a gas injection system 140 for introducing aprocess gas to a remote plasma source 205, wherein the process gascontains an inert gas, and an oxygen-containing gas, or an inert gas anda nitrogen-containing gas. The gas injection system 140 allowsindependent control over the delivery of the process gas to the remoteplasma source 205 from ex-situ gas sources (not shown).

Excited process gas 215 is introduced to the plasma processing region160 from the remote plasma source 205. The excited process gas 215 canbe introduced to the plasma processing region 160 through a gasinjection plenum (not shown), a series of baffle plates (not shown) anda multi-orifice showerhead gas injection plate 165. Optical monitoringsystem 220 can be used to monitor optical emission from the plasmaprocessing region 160. The process chamber 110 is connected to vacuumpump system 150 that can include a turbo-molecular vacuum pump (TMP)capable of a pumping speed up to about 5,000 liters per second (andgreater), and a gate valve for controlling the gas pressure.

Substrate 125 is transferred in and out of process chamber 110 through aslot valve (not shown) and chamber feed-through (not shown) via arobotic substrate transfer system 210 where it is received by substratelift pins (not shown) housed within substrate stage 120 and mechanicallytranslated by devices housed therein. Once the substrate 125 is receivedfrom the substrate transfer system 210, it is lowered to an uppersurface of the substrate stage 120.

The substrate 125 can be affixed to the substrate stage 120 via anelectrostatic clamp (not shown). Furthermore, the substrate stage 120includes a heater element 130 and the substrate stage 120 can furtherinclude a cooling system including a re-circulating coolant flow thatreceives heat from the substrate stage 120 and transfers heat to a heatexchanger system (not shown). Moreover, gas may be delivered to thebackside of the substrate to improve the gas-gap thermal conductancebetween the substrate 125 and the substrate stage 120. Such a system canbe utilized when temperature control of the substrate is required atelevated or reduced temperatures.

A controller 155 includes a microprocessor, a memory, and a digital I/Oport capable of generating control voltages sufficient to communicateand activate inputs to the processing system 100 as well as monitoroutputs from the processing system 100. Moreover, the controller 155 iscoupled to and exchanges information with the process chamber 110, thegas injection system 140, the remote plasma source 205, the opticalmonitoring system 220, the heating element 130, the substrate transfersystem 210, and the vacuum pump system 150. For example, a programstored in the memory can be utilized to control the aforementionedcomponents of a processing system 100 according to a stored processrecipe. One example of controller 155 is a DELL PRECISION WORKSTATION610™, available from Dell Corporation, Austin, Tex.

FIG. 2B shows a plasma processing system for modifying a gate dielectricstack according to an embodiment of the invention. The plasma processingsystem 101 contains a slot antenna 230 that is mounted on a microwavetransmitting window 240. The window 240 can contain Al₂O₃ for efficienttransmission of microwave radiation from external microwave plasmasource 250 into the plasma processing region 160. The microwave powercan, for example, be between about 500 Watts (W) and about 5000 W. Themicrowave frequency can, for example, be 2.45 GHz or 8.3 GHz. The gasinjection system 140 is configured for delivering process gas to theinterior of the process chamber 110 using a gas delivery ring 260located between the window 240 and the substrate 125. The gas deliveryring 260 contains a plurality of gas injection holes 270 for introducingthe process gas 115 into the plasma processing region 160 for excitationby the microwave-powered plasma. In FIG. 2B, the controller is coupledto and exchanges information with the process chamber 110, the gasinjection system 140, the heating element 130, the vacuum pump system150, the substrate transfer system 210, the optical monitoring system220, and the external microwave plasma source 250.

FIG. 2C shows a plasma processing system for modifying a gate dielectricstack according to an embodiment of the invention. The processing system102 of FIG. 2C is capable of forming and sustaining plasma in theprocess chamber 110. In the embodiment shown in FIG. 2C, the substratestage 120 can further serve as an electrode through which radiofrequency (RF) power is coupled to plasma in the plasma processingregion 160. For example, a metal electrode (not shown) in the substratestage 120 can be electrically biased at a RF voltage via thetransmission of RF power from an RF generator 145 through an impedancematch network 135 to the substrate stage 120. The RF bias serves to heatelectrons and, thereby, form and maintain a plasma. A typical frequencyfor the RF bias can range from about 0.1 MHz to about 100 MHz and can beabout 13.6 MHz.

In an alternate embodiment, RF power can be applied to the substratestage 120 at multiple frequencies. Furthermore, the impedance matchnetwork 135 serves to maximize the transfer of RF power to plasma inprocessing chamber 110 by minimizing the reflected power. Match networktopologies (e.g., L-type, π-type, T-type) and automatic control methodsare known in the art. The gas injection system 140 is configured fordelivering process gas 115 to the plasma processing region through amulti-orifice showerhead gas injection plate 165 for excitation by theRF-powered plasma. In FIG. 2C, the controller 155 is coupled to andexchanges information with the process chamber 110, the RF generator145, the impedance match network 1.35, the gas injection system 140, theoptical monitoring system 220, the heating element 130, the substratetransfer system 210, and the vacuum pump system 150.

FIG. 2D shows a plasma processing system for modifying a gate dielectricstack according to an embodiment of the invention. The processing system103 of FIG. 2C further includes either a mechanically or electricallyrotating DC magnetic field system 170 to potentially increase plasmadensity and/or improve plasma processing uniformity, in addition tothose components described with reference to FIG. 2C. Moreover, thecontroller 155 is coupled to the rotating magnetic field system 170 inorder to regulate the speed of rotation and field strength.

FIG. 2E shows a plasma processing system for modifying a gate dielectricstack according to an embodiment of the invention. The processing system104 of FIG. 2E includes, in addition to those components described withreference to FIG. 2C, a multi-orifice showerhead gas injection plate 165that can also serve as an upper plate electrode to which RF power iscoupled from an RF generator 180 through an impedance match network 175.A frequency for the application of RF power to the upper electrode canrange from about 10 MHz to about 200 MHz and can be about 60 MHz.Additionally, a frequency for the application of power to the lowerelectrode (substrate stage 120) can range from about 0.1 MHz to about 30MHz and can be about 2 MHz. Moreover, the controller 155 is coupled tothe RF generator 180 and the impedance match network 175 in order tocontrol the application of RF power to the upper electrode 165.

In one embodiment of the invention, the substrate stage 120 in FIG. 2Ecan be electrically grounded. In an alternate embodiment, a DC bias canbe applied to the substrate stage 120. In still another embodiment, thesubstrate stage 120 can be electrically isolated from the processingsystem 104. In this setup, a floating potential can be formed on thesubstrate stage 120 and on the substrate 125 when the plasma is on.

FIG. 2F shows a plasma processing system for modifying a gate dielectricstack according to an embodiment of the present invention. In additionto those components described with reference to FIG. 2C, the processingsystem 105 of FIG. 2F further includes an inductive coil 195 to which RFpower is coupled via a RF generator 185 through an impedance matchnetwork 190. RF power is inductively coupled from the inductive coil 195through a dielectric window (not shown) to the plasma processing region160. A frequency for the application of RF power to the inductive coil195 can range from about 0.1 MHz to about 100 MHz and can be about 13.6MHz. Similarly, a frequency for the application of power to thesubstrate stage 120 can range from about 0.1 MHz to about 100 MHz andcan be about 13.6 MHz. In addition, a slotted Faraday shield (not shown)can be employed to reduce capacitive coupling between the inductive coil195 and plasma. Moreover, the controller 155 is coupled to the RFgenerator 185 and the impedance match network 190 in order to controlthe application of power to the inductive coil 195.

In one embodiment of the invention, the substrate stage 120 in FIG. 2Fcan be electrically grounded. In an alternate embodiment, a DC bias canbe applied to the substrate stage 120. In still another embodiment, thesubstrate stage 120 can be electrically isolated from the processingsystem 105. In this setup, a floating potential can be formed on thesubstrate stage 120 and on the substrate 125 when the plasma is on.

In another embodiment, the plasma can be formed using electron cyclotronresonance (ECR). In still another embodiment, the plasma can be formedfrom the launching of a Helicon wave. In another embodiment, the plasmacan be formed from a propagating surface wave.

FIGS. 3A and 3B show optical emission (OE) intensity as a function ofwavelength for an oxygen-containing plasma according to an embodiment ofthe invention. A plasma processing system 101 schematically shown inFIG. 2B was utilized to generate plasma from a process gas containing O₂and Ar. FIG. 3A shows an OE feature 300 with maximum intensity at awavelength of about 844.6 nm that is assigned to light emission fromneutral O* radicals in the plasma. Curve 310 shows the measured O*intensity for a process gas pressure of 2 Torr, whereas curve 320 showsthe measured O* intensity for a process gas pressure of 50 mTorr. Theplasma parameters further included an Ar gas flow rate of 2000 standardcubic centimeters per minute (sccm), an O₂ gas flow rate of 200 sccm,and plasma power of 2000 W. FIG. 3A shows that increasing the processgas pressure increases the amount of neutral O* radicals in the plasma.

FIG. 3B shows an OE feature 330 with maximum intensity at a wavelengthbetween about 282 nm and about 283 nm that is assigned to light emissionfrom ionic O₂ ⁺ radicals in the plasma. Curve 340 shows the measured O₂⁺ intensity for a process gas pressure of 2 Torr, whereas curve 350shows the measured O₂ ⁺ intensity for a process gas pressure of 50mTorr. Other plasma parameters were the same as in FIG. 3A. FIG. 3Bshows that increasing the process gas pressure decreases the amount ofionic O₂ ⁺ radicals in the plasma.

In summary, FIGS. 3A and 3B show that the relative amount of neutral O*radicals to ionic O₂ ⁺ radicals in an oxygen-containing plasma can becontrolled over a wide range by varying the process gas pressure. Inparticular, a high process gas pressure allows for generating anoxygen-containing plasma with an increased amount of neutral O* radicalsrelative to the amount of ionic O₂ ⁺ radicals. We estimate that theO*/O₂ ⁺ ratio is about 10 at a low pressure of about 50 mTorr and thatthe O*/O₂ ⁺ ratio is about 114 at a high pressure of about 2 Torr.

FIGS. 4A and 4B show electrical characteristics for a plasma modifiedgate dielectric stack according to an embodiment of the invention. FIG.4A shows the gate current density of a plasma modified gate dielectricstack as a function of gate voltage. Curves 400 and 410 show gateleakage current density (J_(g)) after modifying a HfSiO_(x) high-k layer(˜3 nm thick) with an oxygen-containing plasma generated at high gaspressure (2 Torr), and an oxygen-containing plasma generated at a lowgas pressure (50 mTorr), respectively. FIG. 4A shows that a HfSiO_(x)high-k layer modified using a high pressure oxygen-containing plasma hasnear identical gate current density as a HfSiO_(x) high-k layer modifiedusing a low pressure oxygen-containing plasma.

FIG. 4B shows the capacitance of a plasma modified gate dielectric stackas a function of gate voltage. Curves 420 and 430 show the capacitance(C) of a gate dielectric stack after modifying a HfSiO_(x) high-k layerwith an oxygen-containing plasma generated at a high process gaspressure, and an oxygen-containing plasma generated at a low process gaspressure, respectively. FIG. 4B shows that a HfSiO_(x) high-k layermodified using a high pressure oxygen-containing plasma has a highercapacitance than a HfSiO_(x) layer modified using a low pressureoxygen-containing plasma.

The equivalent oxide thickness (EOT) of the gate dielectric stack thatwas modified using a high-pressure oxygen-containing plasma wasestimated to be about 1.5 nm, whereas the EOT of the gate dielectricstack that was modified using a high-pressure oxygen-containing plasmawas estimated to be about 1.7 nm. The results in FIGS. 4A and 4B showthat a low pressure oxygen-containing plasma modifies a high-k layer bydecreasing the effective dielectric constant of the high-k layer,whereas a high pressure oxygen-containing plasma preserves the thinnessof the interfacial oxide layer, thus maintaining the effectivedielectric constant of the dielectric stack. The inventors believe thatthe high-pressure plasma minimizes the interfacial layer thicknessincrease during the plasma oxidation, reduces the defects in the layer,incorporates oxygen in the layer, removes carbon impurities from thelayer, and yields a lower gate leakage current density than a filmexposed to a low-pressure plasma.

Furthermore, the J_(g) in FIG. 4A are comparable to high-k layers thathave been annealed at high temperatures. Thus, embodiments of thecurrent invention provide a method that can minimize high-temperaturethermal budgets that can increase the thickness of the interfacial oxidelayer.

FIGS. 5A and 5B show OE intensity as a function of wavelength for anitrogen-containing plasma according to an embodiment of the invention.A plasma processing system 101 schematically shown in FIG. 2B wasutilized to generate plasma from a process gas containing N₂ and Ar.FIG. 5A shows an OE feature 500 with maximum intensity at a wavelengthof about 337 nm that is assigned to light emission from neutral N₂*radicals in the plasma. Curve 510 shows the measured N₂* intensity for aprocess gas pressure of 200 mTorr, whereas curve 520 shows the measuredN₂* intensity for a process gas pressure of 800 mTorr, respectively. Theplasma parameters further included an Ar gas flow rate of about 1000sccm, a N₂ gas flow rate of about 10 sccm, and plasma power of 2000 W.FIG. 5A shows that decreasing the process gas pressure from 800 mTorr to200 mTorr decreases the amount of neutral N₂* radicals in the plasma.

FIG. 5B shows an OE feature 530 with maximum intensity at a wavelengthof about 427.2 nm that is assigned to light emission from ionic N₂ ⁺radicals in the plasma. Curve 550 shows the measured N₂ ⁺ intensity fora gas pressure of 800 mTorr, whereas curve 540 shows the measured N₂ ⁺intensity for a process gas pressure of 200 mTorr, respectively. Otherplasma parameters were the same as in FIG. 5A. FIG. 5B shows thatdecreasing the process gas pressure increases the amount of ionic N₂ ⁺radicals in the plasma.

In summary, FIGS. 5A and 5B show that the relative amount of ionic N₂ ⁺radicals to neutral N₂* radicals in a nitrogen-containing plasma can becontrolled over a wide range by varying the process gas pressure. Inparticular, a low process gas pressure allows for generating anitrogen-containing plasma with an increased amount of ionic N₂ ⁺radicals relative to the amount of neutral N₂* radicals.

FIG. 6A shows a nitrogen concentration profile in a gate dielectricstack as a function of plasma conditions and as a function of layerdepth according to an embodiment of the invention. A gate dielectricstack containing a ˜3 nm thick HfSiO_(x) high-k layer deposited onto asubstrate was exposed to a nitrogen-containing plasma generated from aprocess gas containing N₂ and Ar. The Si—N fraction indicates therelative amount of a nitrided interface layer. The Si—N fraction wasmeasured by time-of-flight secondary ion mass spectroscopy (ToF—SIMS)and sputter depth profiling. Curves 610, 620, 630, and 640 show Si—Nfraction in plasma modified gate dielectric stacks for different plasmaconditions. The ratio (R) of ionic nitrogen radicals relative to neutralnitrogen radicals in the plasma decreases from Curve 610 through Curve640.

FIG. 6A shows that a higher amount of ionic nitrogen radicals (i.e.,R₆₁₀) in a plasma resulted in increased nitrogen incorporation into thegate dielectric stack and formation of a thinner nitrided interfaciallayer. The location (depth) of the maximum nitrogen content was found todecrease with increasing amount of ionic radicals in the plasma, asshown by markers 612, 622, and 642, corresponding to maximum intensitiesfor curves 610, 620, and 640, respectively.

FIG. 6B shows nitrogen content in a gate dielectric stack as a functionof plasma exposure time and as function of layer depth according to anembodiment of the invention. The nitrogen content in the gate dielectricstack was found to increase with increasing plasma exposure time.

FIG. 7 is a flowchart for modifying a gate dielectric stack according toan embodiment of the invention. The method uses a plasma process tomodify the gate dielectric stack and improve the properties of thehigh-k layer while minimizing the growth of an interfacial layer on thesubstrate. The process 700 is started at 710. At 720, a gate dielectricstack containing a high-k layer on a substrate is provided. In oneembodiment of the invention, the substrate can contain an interfaciallayer located between the substrate and the high-k layer. At 730, plasmais generated from a process gas containing an inert gas and anoxygen-containing gas, or an inert gas and a nitrogen-containing gas,wherein the process gas pressure is selected to control the amount ofneutral radicals relative to the amount of ionic radicals in the plasma.

In one embodiment of the invention, a plasma is generated at 730 from aprocess gas containing an inert gas and an oxygen-containing gas, wherea high process gas pressure is selected to increase the amount ofneutral oxygen radicals relative to the amount of ionic oxygen radicalsin the plasma. The high-pressure oxygen-containing plasma is capable ofmodifying the gate dielectric stack by increasing the dielectricconstant of the high-k layer through reducing defects in the layer,incorporating oxygen in the layer, and removing carbon impurities fromthe layer.

In another embodiment of the invention, plasma is generated at 730 froma process gas containing an inert gas and a nitrogen-containing gas,where a low process gas pressure is selected to increase the amount ofionic nitrogen radicals relative to the amount of neutral nitrogenradicals in the plasma. The low-pressure nitrogen-containing plasma iscapable of increasing the nitrogen content of the gate dielectric stackand form a thin nitrided interfacial layer.

At 740, the stack is modified by exposing the stack to the high-pressureoxygen-containing plasma or the low-pressure nitrogen-containing plasma.When the plasma exposure has been carried out for the desired amount oftime to modify the stack, the process is ended at 750.

In yet another embodiment of the invention, a gate dielectric stack canbe modified by exposure to a high-pressure oxygen-containing plasma,wherein the plasma contains increased amount of neutral oxygen radicalsrelative to the amount of ionic oxygen radicals in the plasma and,subsequently, the resulting stack can be further modified by exposure toa low-pressure nitrogen-containing plasma, wherein the plasma containsan increased amount of ionic nitrogen radicals relative to the amount ofneutral nitrogen radicals in the plasma. In other words, 730 and 740 areperformed a first time using the high-pressure oxygen-containing plasma,and then 730 and 740 are performed a second time, as indicated by thedashed line in FIG. 7, using the low-pressure nitrogen-containingplasma, and then the process is ended at 750.

In still another embodiment of the invention, a gate dielectric stackcan be modified by exposure to a low-pressure nitrogen-containingplasma, wherein the plasma contains an increased amount of ionicnitrogen radicals relative to the amount of neutral nitrogen radicals inthe plasma and, subsequently, the resulting stack can be furthermodified by exposure to a high-pressure oxygen-containing plasma,wherein the plasma contains increased amount of neutral oxygen radicalsrelative to the amount of ionic oxygen radicals in the plasma. In otherwords, 730 and 740 are performed a first time using the low-pressurenitrogen-containing plasma, and then 730 and 740 are performed a secondtime, as indicated by the dashed line in FIG. 7, using the high-pressureoxygen-containing plasma, and then the process is ended at 750.

As will readily be understood by one skilled in the art, thehigh-pressure oxygen-containing plasma process and the low-pressureoxygen-containing plasma processes described above, can be performedsequentially in the same plasma processing system within a cluster toolor, alternatively, they can be performed in different plasma processingsystems within the same cluster tool. The cluster tool can furthercontain a substrate transfer system configured for transferringsubstrates within the cluster tool, and a controller configured tocontrol the components of the cluster tool.

It should be understood that various modifications and variations of thepresent invention may be employed in practicing the invention. It istherefore to be understood that, within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed herein.

1. A method for modifying a gate dielectric stack, the methodcomprising: providing a gate dielectric stack having a high-k layerformed on a substrate; generating a plasma from a process gas containingan inert gas and one of an oxygen-containing gas or anitrogen-containing gas, and selecting a pressure for the process gaseffective to control the amount of neutral radicals relative to theamount of ionic radicals in the plasma; and modifying the gatedielectric stack by exposing the stack to the plasma, wherein thepressure is selected to increase the amount of neutral radicals relativeto the amount of ionic radicals when the process gas contains anoxygen-containing gas and to increase the amount of ionic radicalsrelative to the amount of neutral radicals when the process gas containsa nitrogen-containing gas.
 2. The method according to claim 1, whereinthe substrate comprises a Si substrate, a Ge-containing Si substrate, aGe substrate, or a compound semiconductor substrate.
 3. The methodaccording to claim 1, wherein the high-k layer comprises a metal oxidelayer or a metal silicate layer.
 4. The method according to claim 1,wherein the high-k layer comprises Ta₂O₅, TiO₂, ZrO₂, Al₂O₃, Y₂O₃,HfSiO_(x), HfO₂, ZrSiO_(x), TaSiO_(x), SrO_(x), SrSiO_(x), LaO_(x),LaSiO_(x), YO_(x), or YSiO_(x), or a combination of two or more thereof.5. The method according to claim 1, wherein the process gas comprises aninert gas and an oxygen-containing gas containing O₂, O₃, H₂O, or H₂O₂,or a combination of two or more thereof.
 6. The method according toclaim 5, wherein the process gas pressure is between about 0.5 Torr andabout 5 Torr.
 7. The method according to claim 1, wherein the processgas comprises an inert gas and a nitrogen-containing gas selected fromN₂ or NH₃, or a combination thereof.
 8. The method according to claim 7,wherein the process gas pressure is between about 10 mTorr and about 400mTorr.
 9. The method according to claim 1, wherein the inert gascomprises He, Ar, He, Kr, or Xe, or a combination of two or morethereof.
 10. The method according to claim 1, wherein the gatedielectric stack further comprises an interfacial layer between thehigh-k layer and the substrate.
 11. The method according to claim 10,wherein the interfacial layer comprises an oxide layer, a nitride layer,or an oxynitride layer.
 12. The method according to claim 1, wherein thegenerating a plasma comprises utilizing a remote plasma source, anexternal microwave plasma source, an inductive coil, a plate electrode,an antenna, an ECR source, a Helicon wave source, or a surface wavesource, or any combination of two or more thereof.
 13. A method formodifying a gate dielectric stack, the method comprising: providing agate dielectric stack having a high-k layer formed on a substrate;generating a first plasma from a first process gas containing a firstinert gas and an oxygen-containing gas, and selecting a pressure for thefirst process gas effective to increase the amount of neutral oxygenradicals relative to the amount of ionic oxygen radicals in the firstplasma; and modifying the gate dielectric stack by exposing the stack tothe first plasma.
 14. The method according to claim 13, wherein thesubstrate comprises a Si substrate, a Ge-containing Si substrate, a Gesubstrate, or a compound semiconductor substrate.
 15. The methodaccording to claim 13, wherein the high-k layer comprises a metal oxidelayer or a metal silicate layer.
 16. The method according to claim 13,wherein the high-k layer comprises Ta₂O₅, TiO₂, ZrO₂, Al₂O₃, Y₂O₃,HfSiO_(x), HfO₂, ZrSiO_(x), TaSiO_(x), SrO_(x), SrSiO_(x), LaO_(x),LaSiO_(x), YO_(x), or YSiO_(x), or a combination of two or more thereof.17. The method according to claim 13, wherein the oxygen-containing gascomprises O₂, O₃, H₂O, or H₂O₂, or a combination of two or more thereof.18. The method according to claim 13, wherein the first inert gascomprises He, Ar, He, Kr, or Xe, or a combination of two or morethereof.
 19. The method according to claim 13, wherein the first processgas pressure is between about 0.5 Torr and about 5 Torr.
 20. The methodaccording to claim 13, wherein the first process gas pressure is betweenabout 1 Torr and about 3 Torr.
 21. The method according to claim 13,wherein the ratio of the first inert gas to the oxygen-containing gas isbetween about 20 and about
 5. 22. The method according to claim 13,wherein the first process gas comprises Ar and O₂.
 23. The methodaccording to claim 22, wherein the Ar/O₂ ratio is between about 20 and5.
 24. The method according to claim 13, further comprising maintainingthe substrate at a temperature between about 150° C. and about 450° C.during the modifying.
 25. The method according to claim 13, wherein themodifying comprises exposing the gate dielectric stack to the firstplasma for a time period between about 5 sec and about 60 sec.
 26. Themethod according to claim 13, wherein the modifying is performed for atime sufficient to increase the effective dielectric constant of thehigh-k layer through at least one of minimizing the interfacial layer,reducing defects in the layer, incorporating oxygen in the layer, orremoving carbon impurities from the layer.
 27. The, method according toclaim 13, wherein the gate dielectric stack further comprises aninterfacial layer between the high-k layer and the substrate.
 28. Themethod according to claim 27, wherein the interfacial layer comprises anoxide layer, a nitride layer, or an oxynitride layer.
 29. The methodaccording to claim 13, further comprising: generating a second plasmafrom a second process gas containing a second inert gas and anitrogen-containing gas, and selecting a pressure for the second processgas effective to increase the amount of ionic nitrogen radicals relativeto the amount of neutral nitrogen radicals in the second plasma; andexposing the modified gate dielectric stack to the second plasma withoutthe first plasma.
 30. The method according to claim 29, wherein thenitrogen-containing gas comprises N₂ or NH₃, or a combination thereof.31. The method according to claim 29, wherein the second inert gascomprises He, Ar, He, Kr, or Xe, or a combination of two or morethereof.
 32. The method according to claim 29, wherein the secondprocess gas pressure is between about 10 mTorr and about 400 mTorr. 33.A method for modifying a gate dielectric stack, the method comprising:providing a gate dielectric stack having a high-k layer formed on asubstrate; generating a first plasma from a first process gas containinga first inert gas and a nitrogen-containing gas, and selecting apressure for the first process gas effective to increase the amount ofionic nitrogen radicals relative to the amount of neutral nitrogenradicals in the first plasma; and modifying the gate dielectric stack byexposing the stack to the first plasma.
 34. The method according toclaim 33, wherein the substrate comprises a Si substrate, a Ge-containedSi substrate, a Ge substrate, or a compound semiconductor substrate. 35.The method according to claim 33, wherein the high-k layer comprises ametal oxide layer or a metal silicate layer.
 36. The method according toclaim 33, wherein the high-k layer comprises Ta₂O₅, TiO₂, ZrO₂, Al₂O₃,Y₂O₃, HfSiO_(x), HfO₂, ZrSiO_(x), TaSiO_(x), SrO_(x), SrSiO_(x),LaO_(x), LaSiO_(x), YO_(x), or YSiO_(x), or a combination of two or morethereof.
 37. The method according to claim 33, wherein thenitrogen-containing gas comprises N₂ or NH₃, or a combination thereof.38. The method according to claim 33, wherein the first inert gascomprises He, Ar, He, Kr, or Xe, or a combination of two or morethereof.
 39. The method according to claim 33, wherein the first processgas pressure is between about 10 mTorr and about 400 mTorr.
 40. Themethod according to claim 33, wherein the first process gas pressure isbetween about 50 mTorr and about 300 mTorr.
 41. The method according toclaim 33, wherein the ratio of the first inert gas to thenitrogen-containing gas is between about 20 and about
 500. 42. Themethod according to claim 33, wherein the first process gas comprises Arand N₂.
 43. The method according to claim 33, wherein the Ar/N₂ ratio isbetween about 20 and about
 500. 44. The method according to claim 33,further comprising maintaining the substrate at a temperature betweenabout 150° C. and about 450° C. during the modifying.
 45. The methodaccording to claim 33, wherein the gate dielectric stack is exposed tothe first plasma for a time period between about 60 sec and about 300sec.
 46. The method according to claim 33, wherein the modifying isperformed for a time sufficient to increase the nitrogen content of thehigh-k layer.
 47. The method according to claim 33, wherein the gatedielectric stack further comprises an interfacial layer between thehigh-k layer and the substrate.
 48. The method according to claim 47,wherein the interfacial layer comprises an oxide layer, a nitride layer,or an oxynitride layer.
 49. The method according to claim 33, furthercomprising: generating a second plasma from a second process gascontaining a second inert gas and an oxygen-containing gas, andselecting a pressure for the second process gas effective to increasethe amount of neutral oxygen radicals relative to the amount of ionicoxygen radicals in the second plasma; and exposing the modified gatedielectric stack to the second plasma without the first plasma.
 50. Themethod according to claim 49, wherein the oxygen-containing gascomprises O₂, O₃, H₂O, or H₂O₂, or a combination of two or more thereof.51. The method according to claim 49, wherein the second inert gascomprises He, Ar, He, Kr, or Xe, or a combination of two or morethereof.
 52. The method according to claim 49, wherein the secondprocess gas pressure is between about 0.5 Torr and about 5 Torr.
 53. Themethod according to claim 49, wherein the second process gas pressure isbetween about 1 Torr and about 3 Torr.